In many instances, customers are seen as one of the key resources for new product development (NPD), as they often have deep product knowledge as well as experience and creativity potential gained by regular product usage. From knowledge management perspective, customers’ input to NPD is manifested in different forms of knowledge. Customers’ input to NPD typically reflects their needs and desires (need information) but may also represent suggestions describing how ideas can be transferred into marketable products (solution information), in some cases it even leads to radical innovations (leading edge information). In order to internalize customer knowledge, in theory different methods are discussed. However, little is known about these methods’ effectiveness and efficiency to transmit customers’ knowledge to firms. This research identifies a total of 15 methods with the help of a systematic literature review. By systematically analyzing these methods, we found that there are methods within which customers are involved only “passively” in NPD, as well as methods that enable a more “active” customer integration. This study exhibits that the methods which enable an active customer integration, compared to methods where customers are integrated only passively in NPD, are more suitable for attaining customer knowledge within innovation development.